To meet customer demand, the commercial printing industry requires the capability of producing spot colors accurately and consistently. Spot colors can be defined as a fixed set of colors which may be Pantone® colors, customer logo colors, colors in a customer's proprietary marked patterns, or customer defined colors in the form of an index color table. Spot colors are often used, or can be used, for large background areas, which may be the most color critical portion of a particular page. Consistent color in these areas may determine the difference between success and failure in meeting customer requirements. Customer demands for color accuracy and consistency are typically much tighter for spot colors than for colors within images.
The color gamut of a printer is a multi-dimensional space of a given volume with the axes of the space being set or defined initially by the pigments used in the colorants of the primary colors such as, for example, cyan, magenta, yellow, and black. In forming multi-color output images on an image-receiving medium, each of the primary colors is transferred to the image-receiving medium in turn. The color gamut is defined by the interaction of the primary colors, and is limited by a total amount of toner in any combination that can be effectively deposited on the image-receiving medium. For each system, the toner deposited on the image-receiving medium, such as a photoreceptor, is referred to as DMA (Developed Mass per unit Area). Otherwise, toner transfer to the image-receiving medium will be affected, and the quality of the produced images will be reduced.
In Image-on-Image printers, the actual available color gamut for a particular image forming device not only depends on colorants of the pigments but it is also a function of DMA. The DMA depends on the set points for certain process parameters of each of the separations. The process parameters are, for example, photoreceptor voltages (charged and/or discharged voltages), donor and/or magnetic roll voltages, toner concentrations and the like.
Combinations of set points affect DMA levels thereby defining limits to the color gamut by limiting the developed color combinations that are available from specific mass levels of each of the primary colorants. These set points are developed in sets, the sets of set points and associated color rendition dictionary (CRD) by which images may be produced by the image forming device. CRDs, and their associated set points, are generally experimentally derived for a given image forming device or system. CRDs are appropriately nominalized in order that the CRD and associated set of set points programmed into an image forming device, or family of image forming devices, substantially ensures that the color gamut produced by that image forming device covers, as broadly as possible, an available nominal color spectrum. Typically then, image forming devices are delivered with a single CRD available in the image production system which is defined by the associated set points adjusted in order to center the color gamut. Such a color gamut is referenced herein as the “normal” color gamut.
Occasionally, customers or other end-users of an individual image forming device, or family of image forming devices, may desire to produce and/or reproduce, on a recurring basis, a color or set of colors that lies outside the nominal color gamut available based on the single CRD that comes pre-stored in the image forming device, i.e. the memory or spot colors. CRDs and individual process parameter set points are not, however, user adjustable. Where a user is provided access to a capability in an image forming device to adjust individual set points, predetermined CRDs would be rendered invalid. A likely outcome would be that the color gamuts available to produce output images would be adversely affected.
Producers and suppliers of image forming devices have a capability to experimentally bias and/or skew the color gamut available in their products. Through experimentation, a new CRD may be defined based on a modified set of set points to adjust the color gamut of an image forming device. In such a case, the total volume of the color gamut available from the image forming device is not increased, but it is shifted or skewed. The library of available colors, however, is modified in order that a different or more vibrant single color-based output set of hues (e.g., vibrant red or vibrant blue) may be made available at the expense of potentially available pallets of other hues, (e.g., green and blue hues) to be produced and/or reproduced within the skewed color gamut defined by this modified color rendition dictionary. Using such a skewed gamut at multiple sets of set points, effective color gamut is increased or extended beyond the nominal gamut.
There is a need for spot color printing with such non-standard gamuts achievable by adjustment of the previous set points when such a spot color is beyond the normal color gamut of the printer as set by the primary color pigment colorants, e.g., cyan, magenta, yellow and black.
It would be advantageous to provide a capability within an image forming device, or family of image forming devices, to adjust the color gamut to encompass a desired spot color by gamut extension on a pixel-by-pixel basis. Such a fractal gamut extension would allow intra page color extensions for improved printer color enhancement and variation. Unfortunately, adjustment of certain printer process parameter set points do not give within page gamut extensions since the changes to mag roll (development roll) bias of process or spot color separations can change the developability of the whole page at a time.
There is thus a need for a method to achieve the extended gamut colors on a pixel-by-pixel for within page fractal gamut extensions and then running the color control and process control loops in a non-standard print mode to achieve the desired extensions.